1. Field of the Invention
The present invention relates to a light source, more particularly to a linear light source suitable for an optical scanning module.
2. Description of the Related Art
An optical scanning module is applied to a scanner, a fax machine, or a multi-function printer (MFP) which incorporates functionality of a photocopier, a scanner, a printer and a fax machine. The optical scanning module usually includes a light source, a reflecting mirror, a focusing lens, and an image sensor.
The light source is capable of emitting light beams for illuminating a scan target. The scan target has a target pattern. The image sensor receives reflected light beams and generates electronic signals corresponding to the target pattern. The aforementioned light source generally adopts a cold cathode fluorescent lamp (CCFL) for emitting white light, and illuminates the scan target via an elongated slit. The reflected light beams from the scan target are reflected once again by the reflecting mirror and are focused by the focusing lens so as to be imaged on the image sensor. However, since the CCFL requires an inverter for providing high-voltage and high-frequency alternating current to operate, an issue of high power consumption is presented. Moreover, mercury vapor filled in a lamp tube of the CCFL may pollute the environment, such that use of the CCFL is regulated in many countries.
Therefore, in recent years, optical scanning modules adopting a linear light source which is formed by a light emitting diode (LED) in cooperation with a light guide as the light source have been proposed. Referring to FIG. 1, the conventional linear light source 9 includes a light guide 91, a LED package 92 disposed at one end of the light guide 91, and a reflecting shield 93 covering side walls of the light guide 91. A transverse section of the light guide 91 is rectangular or polygonal in shape. The light guide 91 has a light-exit surface 911, and is provided with a plurality of optical structures 912 disposed on a surface of the light guide 91 opposite to the light-exit surface 911. In this design, since light beams exiting the light-exit surface 911 are divergent light beams, aside from the light beams illuminating the scan target, the other light beams are wasted.
Moreover, a light guide of a linear light source of an optical scanning module nowadays have a common issue, that is, light beams emitted from the linear light source are merely concentrated at a specific distance away from the light guide. For example, a conventional linear light source including a light guide which has a hexagonal cross section is designed to concentrate light beams emitted therefrom at a position about 0.4 mm above a transparent plate. Once a target to be scanned substantially deviates from the position at which the light beams are concentrated, illumination of the target is significantly reduced. Therefore, in a condition that paper to be scanned has uneven surfaces or wrinkles, the conventional linear light source may not evenly illuminate the paper, resulting in inferior scanning quality.
It is apparent from the foregoing that light beams emitted from the linear light source of the conventional optical scanning module are merely concentrated at a specific position. Accordingly, this invention attempts to solve this issue so as to promote scanning quality.